[PATCH 6/5] KVM: x86: fix periodic lapic timer with hrtimers

["Radim Krčmář" <rkrcmar@redhat.com>]

I have only compile-tested it, but it should optimize the switch and
also fix two bugs.  The first one is major.
This needs that the deadline clearing in [5/5] is fixed.
---8<---
We must start the hrimer even if the expiration is already in the past,
otherwise the periodic timer would not rearm the hrtimer.

And computing next expiration of a period timer does not require current
time.  The period should be constant, so it is more precise to add the
period to the last expiration time.  This fixes a time difference
between hrtimer expiration and apic->lapic_timer.target_expiration.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
---
 arch/x86/kvm/lapic.c | 38 +++++++++++++-------------------------
 1 file changed, 13 insertions(+), 25 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index f4734fe12803..6244988418be 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1350,19 +1350,19 @@ static void start_sw_tscdeadline(struct kvm_lapic *apic)
 
 static void start_sw_period(struct kvm_lapic *apic)
 {
-	ktime_t now;
-
-	now = apic->lapic_timer.timer.base->get_time();
-
 	if (!apic->lapic_timer.period)
 		return;
 
-	if (likely(ktime_after(apic->lapic_timer.target_expiration, now)))
-		hrtimer_start(&apic->lapic_timer.timer,
-			apic->lapic_timer.target_expiration,
-			HRTIMER_MODE_ABS_PINNED);
-	else
+	if (apic_lvtt_oneshot(apic) &&
+	    ktime_after(apic->lapic_timer.target_expiration,
+	                apic->lapic_timer.timer.base->get_time())) {
 		apic_timer_expired(apic);
+		return;
+	}
+
+	hrtimer_start(&apic->lapic_timer.timer,
+		apic->lapic_timer.target_expiration,
+		HRTIMER_MODE_ABS_PINNED);
 }
 
 static bool set_target_expiration(struct kvm_lapic *apic)
@@ -1414,10 +1414,11 @@ static bool set_target_expiration(struct kvm_lapic *apic)
 
 static void advance_periodic_target_expiration(struct kvm_lapic *apic)
 {
-	apic->lapic_timer.tscdeadline = kvm_read_l1_tsc(apic->vcpu, rdtsc()) +
+	apic->lapic_timer.tscdeadline +=
 		nsec_to_cycles(apic->vcpu, apic->lapic_timer.period);
-	apic->lapic_timer.target_expiration = ktime_add_ns(apic->lapic_timer.timer.base->get_time(),
-		apic->lapic_timer.period);
+	apic->lapic_timer.target_expiration =
+		ktime_add_ns(apic->lapic_timer.target_expiration,
+				apic->lapic_timer.period);
 }
 
 bool kvm_lapic_hv_timer_in_use(struct kvm_vcpu *vcpu)
@@ -1481,19 +1482,6 @@ void kvm_lapic_switch_to_hv_timer(struct kvm_vcpu *vcpu)
 
 	WARN_ON(apic->lapic_timer.hv_timer_in_use);
 
-	if (apic_lvtt_period(apic)) {
-		ktime_t remaining, now;
-		u64 tscl = rdtsc();
-
-		now = apic->lapic_timer.timer.base->get_time();
-		remaining = hrtimer_get_remaining(&apic->lapic_timer.timer);
-		if (ktime_to_ns(remaining) < 0)
-			remaining = ktime_set(0, 0);
-
-		apic->lapic_timer.tscdeadline = kvm_read_l1_tsc(apic->vcpu, tscl) +
-			nsec_to_cycles(apic->vcpu, ktime_to_ns(remaining));
-		apic->lapic_timer.target_expiration = ktime_add_ns(now, ktime_to_ns(remaining));
-	}
 	start_hv_timer(apic);
 }
 EXPORT_SYMBOL_GPL(kvm_lapic_switch_to_hv_timer);
-- 
2.10.1